1. Field of the Invention
The present invention relates to an optical guide plate used for a backlight source of a liquid crystal display apparatus. In particular, the present invention relates to a diffusion optical guide plate which can control the viewing angle characteristic of a liquid crystal display apparatus. Furthermore, the present invention relates to a backlight source and a liquid crystal display apparatus using such a diffusion optical guide plate.
2. Description of the Related Art
In general, a liquid crystal display apparatus has widely been used for numerical segment-type display apparatus such as watches, electric calculators, and the like. A liquid crystal display device which is incorporated in the liquid crystal display apparatus comprises a transparent substrate on which an active element such as a thin film transistor (TFT) is formed as a means for switching the pixel electrodes which apply voltage to the liquid crystal. The liquid crystal display apparatus further comprises a color (such as red, green, blue, etc.) filter layer as a color display means when a color display is performed.
Depending on the twist angle of the liquid crystal molecules, known display modes of a liquid crystal display apparatus include: (1) an active driving-type twisted nematic (TN) liquid crystal display mode which includes liquid crystal molecules twistedly aligned at an angle of 90.degree.; and (2) a multiplex driving-type super twisted nematic (STN) liquid crystal display mode which includes liquid crystal molecules twistedly aligned at an angle of more than 90.degree. and utilizing the steepness of the transmittance-applied voltage characteristics.
Since these liquid crystal display apparatus are characterized in that they are thin, light weight and consume little electric power, they have rapidly been developed in display fields (e.g., word processors, notebook-type personal computers, portable TVs, car navigation TVs, and the like) in recent years. In the future, because various images are expected to be widely used in public with the development of these portable liquid crystal display apparatus, it will be more and more important to protect personal privacy regarding these images.
According to the conventional liquid crystal display apparatus, as shown in FIG. 8, the display image of a liquid crystal display apparatus 28 can be recognized by not only an observer 29 located directly in the front of the display, but by other observers 30 and 31 located to either side (a wide viewing angle display mode). Therefore, as shown in FIG. 9, it is desirable that the viewing angle (2.theta.) of the liquid crystal display apparatus 28 be narrow compared to that shown in FIG. 8 for the purpose of protecting the user's personal privacy regarding the images being viewed, by providing a liquid crystal display apparatus having a structure so that only the observer 29 can clearly recognize the display image (a narrow viewing angle display mode). In other words, it is desirable that the viewing angle characteristic of the liquid crystal display apparatus can readily be varied depending upon whether the image is displayed to a large number of people, or to only an observer located directly in front of the display.
In order to respond this demand, Japanese Laid-open Patent Publication No. 5-108023 proposes a liquid crystal display apparatus having a controllable viewing angle. The liquid crystal display apparatus comprises a light shutter TN liquid crystal cell attached to a display TN liquid crystal cell. Specifically, a polarizing plate, a light shutter TN liquid crystal cell and an optical path controlling plate are formed on the light source side of the display TN liquid crystal cell, while another polarizing plate is formed on its opposite side. The display image of the display TN liquid crystal cell can be either a wide viewing angle mode or a narrow viewing angle mode for preventing viewing by observers other than the user, by switching between the open and close conditions of the light shutter TN liquid crystal cell.
However, this liquid crystal display apparatus which has a controllable viewing angle requires two liquid crystal cells (i.e., the display liquid crystal cell and the light shutter liquid crystal cell). Therefore, it is difficult for such a liquid crystal display apparatus to be fabricated to have a thin shape. Furthermore, fabrication of the liquid crystal display apparatus is complicated and provides for less efficient productivity. In addition, the liquid crystal display apparatus further requires a polarizing plate incorporated in the light shuttering TN cell, and thus has a total of three polarizing plates. Therefore, a problem arises in that the liquid crystal display apparatus provides reduced light transmittance, compared to the conventional TN and STN liquid crystal display apparatus.
Japanese Laid-open Patent Publication No. 5-72529 discloses a liquid crystal display apparatus having reduced viewing angle dependency of its characteristics. The liquid crystal display apparatus comprises an additional liquid crystal layer for controlling light scattering on one side of one of the polarizing plates, in addition to the conventional liquid crystal display device having a display mode utilizing optical rotation or birefringence (such as those of TN-mode or STN-mode, homogeneous-mode, or the like).
As described above, this liquid crystal display device requires the use of an additional liquid crystal (e.g., DSM type liquid crystals, polymer dispersed type liquid crystals, etc.) as a liquid crystal for controlling light scattering. This requirement causes an increase in production cost. The liquid crystal display apparatus which uses a scattering mode provides a very low brightness of its display image viewed from the direction vertical to the display screen, because a light transmitting the liquid crystal display apparatus is scattered at a wide angle. Thus, the liquid crystal display apparatus which uses a scattering mode cannot display the image clearly.